The present invention relates to a brake system with slip control for automotive vehicles with front-wheel or rear-wheel drive. The system comprises a pedal-actuated, preferably auxiliary-force-assisted braking pressure generator, to which in each case the wheel brakes of one front wheel and one rear wheel are connected by way of two separate brake lines. An auxiliary-pressure hydraulic supply system having a hydraulic pump, and a pressure-compensating and pressure-fluid supply reservoir are provided. An auxiliary-pressure control valve is provided. Wheel sensors and electronic circuits are provided for determining the wheel rotational behavior and for generating electric braking-pressure control signals which, for the purpose of slip control, serve to control electromagnetically actuatable pressure-fluid inlet valves and outlet valves inserted into the pressure-fluid lines.
It is known that the locking of the vehicle wheels during a braking operation can be prevented in hydraulic or compressed-air operated brake systems by means of so-called inlet valves and outlet valves which are inserted into the pressure-fluid conduit to the wheel brakes. The valves are also inserted into a return line or into a pressure-discharge conduit which precludes further braking pressure rise or reduces the braking pressure as soon as the deceleration and/or the wheel slip exceeds a predetermined value. Systems of this type are known in a number of variants.
An optimal braking behavior, that is, high driving stability and directional stability and short stopping distance, can be expected at each vehicle wheel when the braking pressure is controlled individually. However, since the expenditure needed for component parts including controlling and regulating circuits, monitoring and safety circuitries etc., is very high, and since limitation to two or three control channels in combination with specific selection criteria such as select-low, select-high and with a concurrent control of the braking pressure in several wheel brakes is not disadvantages, it is expedient to depart from the individual control of all wheels.
Therefore, dual-circuit hydraulic brake systems of the type referred to have been developed and described which, on detection of an imminent risk of wheel lock, vary the braking pressure concurrently. That is, jointly for the two wheels connected by means of one common pair of inlet valve and outlet valves. If, to this end, the pressure in each control channel is dimensioned according to the select-low principle following the wheel with the worst road contact, a longer stopping distance must be tolerated.
Furthermore, it is known to additionally insert a two-way/two-position directional control valve into the pressure-fluid conduit to the wheel brake, so that, when the rear wheel is relieved from load due to the dynamic axle load shift, after change-over of this valve, locking of the relieved rear wheel is avoided. Simultaneously, further braking pressure rise at the front wheel can be effected (German published patent application No. 31 36 616). In situations in which it is decisive to use the braking effect of the rear wheels, particularly in the presence of a low road adherence valve, this known measure will not afford any improvement.
It has been proposed to normally derive the reference value for dimensioning the braking pressure in the diagonals from the front wheel, but to perform a change-over in specific situations according to predetermined criteria so that temporarily the rear wheel instead of the front wheel takes on the lead. That is, the rear wheel determines the braking pressure (German patent application No. 33 14 802.3). In some situations, however, it is a disadvantage to jointly control the braking pressure of the two wheels connected to one pressure-fluid circuit in comparison to systems with three or four control channels.
It is an object of the present invention to eliminate the shortcomings of the known slip-controlled dual-circuit brake systems and to devise a brake system which requires minor manufacturing effort and, nevertheless, affords an effective controlled braking operation both in the presence of high and low frictional values, while driving stability and steerability are preserved in all important situations. More specifically, the brake system is to be suitable for vehicles with a diagonal brake circuit split-up, and wherein braking is desired even in the event of the two wheels of the driven vehicle axle reaching the slip range as a result of the engine trailing torque, while the two wheels of the non-driven vehicle axle roll freely.